Multilayered structures formed from two or more polymeric layers are well known in the art and are typically fabricated for the purpose of obtaining a single structure having some of the advantages of each of the separate layers. Processes that are typically used to form multilayered polymeric structures include lamination, extrusion, coextrusion, blow molding, and injection molding. Before such multilayered structures will perform successfully on an ongoing basis, however, it is important that the various layers be suitably adhered one to the other so as to avoid separation (i.e., delamination) during use. If the layers of the structure do not adhere sufficiently to each other, one or more tie layers formed from adhesive compositions have been used to join certain types of layers of a multilayer structure together. These multilayered structures find use in a variety of applications in the building and construction markets; in packaging; in leisure and recreation applications; and in the automotive industry.
Methods for increasing the adhesion between certain types of layers have been reported, for example, U.S. Pat. No. 4,588,648 discloses an adhesive layer containing an olefin copolymer grafted with maleic anhydride. U.S. Pat. No. 5,306,548 teaches the need for an intermediate adhesive layer which ties or bonds a weatherable outer layer and a polyolefin substrate such as polyethylene. The weatherable outer layer is a blend of acrylonitrile-ethylene/propylene rubber-styrene graft copolymers and a thermoplastic elastomer such as chlorinated polyethylene, aliphatic urethane, saturated styrenic block copolymers, and ethylene propylene diene copolymers. The coextruded adhesive layer contains chlorinated polyethylene, styrenic diblock and triblock copolymers, copolyamide adhesives, polyester adhesives, polyurethane adhesives, PVC, and mixtures thereof.
In addition to initial adhesion problems, however, dissimilar materials may also exhibit processability issues due to differences in rheological properties. At typical processing temperatures, polymeric materials with similar shear viscosities may have significantly different responses to the forces of tensile deformation and extensional flow. Such viscoelastic differences may result in appearance and processing problems (i.e., melt fracture, slip-and-stick phenomenon, sharkskin, and other irregular surface imperfections), as well as variations in the thickness of the layers. For example, coextruding an adhesive tie layer containing greater than 95 percent by weight of styrenic block copolymer along with a second, non-adhesive layer may result in a sharkskin appearance in the finished part and a non-uniform thickness of the adhesive layer, which detracts significantly from the aesthetics of the multilayered structure. If the processing incompatibility is severe, the layers may separate completely during processing or even later during use due to inadequate adhesion.
Blending a styrenic block copolymer with a second component to form an adhesive tie layer has also been reported. Such attempts to improve the viscoelastic differences between the various layers, however, can disadvantageously result in a reduction in the adhesive properties of the tie layer. WO 2003/046102, for example, discloses a tie layer composition containing maleic anhydride grafted styrene-ethylene-butylene-styrene block copolymer and at least one adhesion promoter, such as a tackifier resin. The material forming the tie layer may be dissolved in a suitable organic solvent and applied as a solution onto the surface of a substrate layer. EP Patent No. 994776 B1 discloses a multilayer film with an intermediate tie layer composed of a blend of a styrene-ethylene-butylene-styrene block copolymer and either a functionalized polyolefin or an ethylene acrylate copolymer.
Multilayered polymeric structures containing acrylate-based cap layers and polyolefin-based substrate layers have also been used. The cap layer provides a glossy, scratch resistance outer layer with good optical characteristics while the polyolefin-based substrate provides a layer with a balance of stiffness and impact resistance, excellent processability, and low cost. EP Patent No. 651013 B1, for example, discloses a process for improving the adherence between (meth)acrylic films and polyolefin substrates through the use of a tie layer containing 5-95 parts by weight of a butyl acrylate/methyl methacrylate core-shell polymer and 5-95 parts by weight of a grafted polypropylene/poly(methyl methacrylate/butyl acrylate) copolymer.
In addition, U.S. Pat. No. 6,455,171 relates to a multilayer structure containing a cap layer, a substrate layer, and a tie layer including either an olefin acrylate or a vinyl aromatic diene block copolymer. The cap layer includes at least 50 percent by weight of a polymethacrylate ester or polyacrylate ester. The substrate layer preferably includes 30 to 70 percent by weight propylene polymer containing less than 10 percent by weight ethylene, 7 to 65 percent by weight of a polymer of a aromatic vinyl monomer (i.e., polystyrene), and 5 to 20 percent by weight vinyl aromatic diene block copolymer. The tie layer preferably includes a multiblock polymer of styrene and either isoprene or butadiene in a particular ratio (e.g., 43% styrene:57% butadiene). In one embodiment, the tie layer contains an olefin acrylate such as ethylene methyl acrylate, ethylene ethyl acrylate, or ethylene butyl acrylate.
Polymeric materials containing unsaturated elastomers are generally, however, not thermally stable. Thus, it tends to be difficult to use them at temperatures at or above the temperature at which degradation of the unsaturated elastomer occurs, thereby limiting the usefulness of such materials in processes such as coextrusion. It is further known that unsaturated elastomers, such as conjugated diene rubbers, which have unsaturated bonds in the main chain as opposed to the side chains, are more susceptible to damage from ultraviolet radiation, oxygen, and ozone (i.e., poor weatherability). In addition, when unsaturated conjugated dienes such as butadiene are used in pigmented polymeric composites, color retention after significant outdoor exposure may be unacceptable (i.e., fading).
In light of these deficiencies, therefore, there is a need for tie layer compositions that can be used to adhere polyolefin-based substrates in particular to form multilayered polymeric structures, preferably with a tie layer disposed adjacent to the substrate layer(s) that permits the multilayer product to be prepared, finished and used without severe temperature and weathering limitations typically required of conventional tie materials. Although the multilayered polymeric structures described in the prior art above each has one or more specific properties that may exceed the norm, including mechanical strength, toughness, and scratch resistance, there remains a need for multilayered polymeric structures having greater adhesion between the layers, better weatherability, and increased color retention, while having good or increased processability and aesthetics.